1. Field of the Invention
The present invention relates to a heat-displacing T-die used in an extruding machine for films, sheets, etc., and more particularly, to an improvement of a heat-displacing T-die designed so that a die slit gap formed between lips is adjusted by utilizing thermal expansion and shrinkage of die bolts.
2. Information of the Related Art
A T-die of this type has a pair of lips, fixed and movable or flexible. Die bolts are connected to the flexible lip in order to adjust a die slit gap formed between the fixed and flexible lips. The die bolts are used to adjust the die slit gap, thereby regulating the thickness of a film or the like to be formed.
Conventional T-dies include ones which are designed so that the die slit gap is automatically regulated by controlling the thermal expansion and shrinkage of heat-displacing actuators (e.g., Jpn. UM Appln. KOKOKU Publication No. 5-40993), besides ones in which die bolts are operated manually.
FIG. 5 shows a conventional T-die which is provided with reverse-acting actuators. A flexible lip 62 and a fixed lip 63 are formed on the distal ends of an A-die body 60 and a B-die body 61, respectively. A die slit gap 64 is defined by the respective opposite faces of the lips 62 and 63.
In the A-die body 60 having the flexible lip 62, a mounting flange 65 projects over the lip 62, and one end of a heat reverse-acting actuator 66 is fixedly screwed to the flange 65.
Basically, the heat reverse-acting actuator 66 is composed of a heat sleeve 68, which is coaxially penetrated by a die bolt 67, and an adjustable heater 69 fitted on the sleeve 68 so as to be intimately in contact with the outer peripheral surface thereof.
In this case, the T-die is a push-only die, and the distal end of the die bolt 67 penetrates a guide 71 for guiding it in movement relative to the heat sleeve 68, abuts against the flexible lip 62. An adjust screw 70 is connected to the proximal end portion of the die bolt 67. The force of pressure of the die bolt 67 can be initially adjusted by manual operation such that the screw 70 is screwed into the free end portion of the heat sleeve 68. Low-thermal-expansion coefficient cast iron or engineering ceramics, such as silicon carbide, is used as the material of the die bolt 67. On the other hand, the heat sleeve 68 and the adjust screw 70 are formed of steel and so on. The material of the sleeve 68 has a higher coefficient of thermal expansion.
In this reverse-acting actuator, the materials of the die bolt 67 and the heat sleeve 68 have different coefficients of thermal expansion, so that the free end portion of the sleeve 68 is extended upward by heat. Since the die bolt 67 is formed of low-thermal-expansion coefficient cast iron or engineering ceramics, its thermal expansion is lower than that of the heat sleeve 68. Owing to this difference in thermal expansion, the upper surface of its proximal flange and the lower surface move upward. Thereupon, the load of pressure on the flexible lip 62 is reduced, so that the die slit gap 64 can be widened. If the heat sleeve 68 is cooled, in contrast with this, the heat sleeve 68 shrinks to a higher degree than the die bolt 67. Accordingly, the die bolt 67 relatively acts so as to press down the flexible lip 62, whereupon the die slit gap 64 is narrowed.
Disposed on the downstream side of the T-die, therefore, is an infrared thickness indicator for use as a detector for measuring the thickness of the extruded film or the like. The film thickness can be automatically adjusted to a constant value by controlling the temperature of the heater 69 for heating the heat sleeve 68 in accordance with the result of the measurement by means of the thickness indicator.
As modern practical-use automatic T-dies of the heat displacing type, there are ones which are provided with push-and-pull heat-displacing actuators adapted to displace flexible lips in both push- and pull-directions, besides the push-only die shown in FIG. 5, and ones which are provided with a mechanism capable of automatically adjusting the die slit gap 64 by means of heat-displacing actuators with a fixed-side lip entirely or partially flexible.
In the case of the die slit gap adjustment by means of the heat-displacing actuators, however, the manipulated variable is small and the thermal expansion or shrinkage is low, so that the range of adjustment is narrow. Accordingly, the operation for the initial adjustment of the die slit gap is particularly troublesome.
In both pushing and pulling the flexible lip, in particular, the heater temperature must pass the dead zone as it changes covering both the heating and cooling sides, due to backlash between screw sections in engagement with the die bolt and the heat sleeve. Thus, the controllability is lowered considerably.